In recent years, with an increase in traffic, a submarine cable system is demanded to have a wide-band circuit (line) or to have a network having a high functionality. A technique such as an OADM (Optical Add-Drop Multiplexer) or an ROADM (Reconfigurable Optical Add-Drop Multiplexer) is therefore applied to a submarine cable system.
A submarine ROADM system uses a wavelength division multiplexing (WDM: Wavelength Division Multiplexing) communication. In a submarine ROADM system, for example, a transmit device inputs a client signal as a wavelength multiplexed optical signal in a submarine cable to accommodate a plurality of paths in one optical fiber, thereby improving flexibility of a network.
In a submarine cable system having an OADM function, the total power of a signal transmitted in a cable composed of optical fibers is set to be constant. When a part of wavelength components of a signal disappears by a breakage of the cable or the like, the signal amplifies other wavelength components, thereby keeping the total power of the signal constant.
By increasing only the power of a specific wavelength component of a signal up to a predetermined value or larger, however, optical spectrum changes caused by deterioration of a waveform of the signal or the like by a nonlinear effect of an optical fiber, thereby deteriorating the transmission quality of the signal.
An optical communication system described in PTL 1 discloses a technique in which, when a failure occurs in a cable, the total power of a signal is corrected by a dummy light to secure communication quality. In an optical communication system described in PTL 1, a terminal equipment (optical transmission device) comprises a dummy light generating unit which generates a dummy light corresponding to a location where an optical signal break has occurred when a cable break failure is generated, and keeps the intensity (power) of a signal to be transmitted constant. PTL 2 describes a configuration for enhancing a security function of a signal wavelength.